Separation membrane structure

ABSTRACT

A separation membrane structure comprises a porous support, a first separation membrane formed on the porous support, and a second separation membrane formed on the first separation membrane. The first separation membrane has an average pore diameter of greater than or equal to 0.32 nm and less than or equal to 0.44 nm. The second separation membrane includes addition of at least one of a metal cation or a metal complex that tends to adsorb nitrogen in comparison to methane.

TECHNICAL FIELD

The present invention relates a separation membrane structure configuredto separate methane and nitrogen.

BACKGROUND ART

Various methods have been proposed for the purpose of separating methaneand nitrogen.

For example, there has been proposal of a means for adsorption andremoval of nitrogen by use of a pressure swing adsorption method using amolecular sieve of activated carbon (reference is made to PatentLiterature 1), or a method of adsorption and removal of nitrogen by apressure swing adsorption method using ETS-4 in which cations areexchanged to barium (reference is made to Patent Literature 2).

Furthermore a means for separating nitrogen by a membrane separationmethod respectively using a CHA-type zeolite membrane, a DDR typezeolite membrane or an organic membrane is also known (reference is madeto Non-Patent Literature 1 to 3).

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Application Laid-Open No.    2000-312824-   [Patent Literature 2] Published Japanese Translation No. 2001-526109    of the PCT International Application

Non-Patent Literature

-   [Non-Patent Literature 1] Ting Wu et al. (6 others), “Influence of    propane on CO₂/CH₄ and N₂/CH₄ separations in CHA zeolite membranes”    Journal of Membrane Science, 473 (2015) 201-209.-   [Non-Patent Literature 2] J. van den Bergh et al. (4 others),    “Separation and permeation characteristics of a DD3R zeolite    membrane”, Journal of Membrane Science, 316 (2008) 35-45.-   [Non-Patent Literature 3] Lloyd M. Robeson, “The upper bound    revisited”, Journal of Membrane Science, 320 (2008) 390-400.

SUMMARY OF INVENTION Technical Problem

However, the means discussed above have not reached sufficientseparation performance since the molecular diameter of methane is closeto the molecular diameter of nitrogen.

The present invention is proposed based on the new insight above, andhas the object of providing a separation membrane structure that canefficiently separate methane and nitrogen.

Solution to Problem

The separation membrane structure according to the present inventioncomprises a porous support, a first separation membrane formed on theporous support, and a second separation membrane formed on the firstseparation membrane. The first separation membrane has an average porediameter of greater than or equal to 0.32 nm and less than or equal to0.44 nm. The second separation membrane includes addition of at leastone of a metal cation or a metal complex that tends to adsorb nitrogenin comparison to methane.

Advantageous Effects of Invention

According to the present invention, it is possible to provide aseparation membrane structure that is configured to efficiently separatemethane and nitrogen.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a separation membrane structure.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below makingreference to the FIGURES. Those aspects of configuration in thefollowing description of the FIGURES that are the same or similar aredenoted by the same or similar reference numerals. However, the FIGURESare merely illustrative, and the actual ratios or the like of therespective dimensions may differ.

Configuration of Separation Membrane Structure 10

FIG. 1 is a cross-sectional view of a separation membrane structure 10.The separation membrane structure 10 enables selective permeation ofnitrogen in a mixed gas that contains at least methane molecule(referred to below as “methane”) and nitrogen molecule (referred tobelow as “nitrogen”). The separation membrane structure 10 includes aporous support 20, a first separation membrane 30, and a secondseparation membrane 40.

The porous support 20 supports the first separation membrane 30 and thesecond separation membrane 40. The porous support 20 exhibits chemicalstability that enables the formation (crystallization, coating, orprecipitation) in sequence of the first separation membrane 30 and thesecond separation membrane 40 in a membrane configuration on a surface.

The porous support 20 may be configured in a shape to enable supply amixed gas that contains at least methane and nitrogen to the firstseparation membrane 30 and the second separation membrane 40. The shapeof the porous support 20 for example may be configured in a honeycomb,monolithic, flat, tubular, cylindrical, columnar, square column shape,or the like.

The porous support 20 according to the present embodiment has asubstrate 21, an intermediate layer 22 and a surface layer 23.

The substrate 21 is configured from a porous material. The porousmaterial includes for example, a sintered ceramic, a metal, an organicpolymer, glass, carbon or the like. The sintered ceramic includesalumina, silica, mullite, zirconia, titania, yttria, silicon nitride,silicon carbide, or the like. The metal includes aluminum, iron, bronze,stainless steel, or the like. The organic polymer includes polyethylene,polypropylene, polytetrafluoroethylene, polysulfone, polyimide or thelike.

The substrate 21 may include an inorganic binder. The inorganic bindermay include use of at least one of titania, mullite, sinterable alumina,silica, glass frits, clay minerals, and sinterable cordierite.

The average particle diameter of the substrate 21 may be configured as 5microns to 25 microns. The average pore diameter of the substrate 21 canbe measured using a mercury porosimeter. The porosity of the substrate21 may be configured for example as 25% to 50%. The average particlediameter of the porous material that configures the substrate 21 may beconfigured for example as 5 microns to 100 microns. In the presentembodiment, the term “average particle diameter” denotes the value ofthe arithmetic mean for the maximum diameter of 30 measured particlesthat are measured by cross sectional micro-structure observation by useof a scanning electron microscope (SEM).

The intermediate layer 22 is formed on the substrate 21. Theintermediate layer 22 can be configured by the porous material that canbe used in the substrate 21. The average pore diameter of theintermediate layer 22 may be smaller than the average pore diameter ofthe substrate 21, and may be configured for example as 0.005 microns to2 microns. The average pore diameter of the intermediate layer 22 may bemeasured by a perm-porometer. The porosity of the intermediate layer 22may be configured as 20% to 60%. The thickness of the intermediate layer22 may be configured for example as 30 microns to 300 microns.

The surface layer 23 is formed on the intermediate layer 22. The surfacelayer 23 can be configured by the porous material that can be used inthe substrate 21. The average pore diameter of the surface layer 23 maybe smaller than the average pore diameter of the intermediate layer 22,and may be configured for example as 0.001 microns to 1 micron. Theaverage pore diameter of the surface layer 23 may be measured by aperm-porometer. The porosity of the surface layer 23 may be configuredto 20% to 60%. The thickness of the surface layer 23 for example may beconfigured as 1 micron to 50 microns.

The first separation membrane 30 is formed on the porous support 20(more specifically, on the surface layer 23). The first separationmembrane 30 can be configured by an inorganic material, an organicmaterial, a metal material, or a composite material of such materials.In consideration of thermal resistance properties and organic solventresistant properties, an inorganic membrane such as a zeolite membrane,silica membrane or carbon membrane is suitable as the first separationmembrane 30, and a zeolite membrane is more preferred in light of thetendency to form a narrow pore diameter distribution. It is noted that asilica membrane includes an organic silica membrane in which an organicfunctional group is bound to silica.

The average pore diameter of the first separation membrane 30 is greaterthan or equal to 0.32 nm and less than or equal to 0.44 nm. Therefore,the first separation membrane 30 allows permeation of nitrogen (dynamicmolecular diameter: about 0.36 nm) that flows from the side with thesecond separation membrane 40 and inhibits permeation of methane(dynamic molecular diameter: about 0.38 nm). Therefore the firstseparation membrane 30 enables a configuration that is termed a“nitrogen selective permeation layer” that enables selective permeationof nitrogen from a mixed gas that contains at least methane andnitrogen. The average pore diameter of the first separation membrane 30is preferably greater than or equal to 0.33 nm and more preferably lessthan or equal to 0.43 in consideration of achieving both a satisfactoryseparation performance and a permeation rate.

A narrow pore diameter distribution in the first separation membrane 30is preferred. That is to say, it is preferred that there is a smallvariation in the pore diameter of the first separation membrane 30. Inthis manner, it is possible to further enhance the nitrogen separationperformance of the separation membrane structure 10. More specifically,a variation coefficient obtained by dividing the standard deviation ofthe pore diameters of the first separation membrane 30 by the averagepore diameter is preferably less than or equal to 0.4, and morepreferably less than or equal to 0.2. The variation coefficient of thepore diameter is a representative value that expresses the degree ofvariation in the pore diameter distribution. Membrane permeation bymethane can be inhibited by reducing the proportion of pores having alarge diameter by a configuration in which the variation coefficient isless than or equal to 0.4.

Although there is no particular limitation in relation to the thicknessof the first separation membrane 30, it may be configured for example as0.1 micron to 10 microns. When the first separation membrane 30 has athick configuration, there is a tendency for nitrogen separationperformance to be enhanced, and when the first separation membrane 30has a thin configuration, there is a tendency for the nitrogenpermeation rate to increase.

When the first separation membrane 30 is a zeolite membrane, althoughthere is no particular limitation in relation to the framework structure(type) of the zeolite, for example, ABW, ACO, AEI, AEN, AFN, AFT, AFV,AFX, APC, ATN, ATT, ATV, AVL, AWO, AWW, BIK, BRE, CAS, CDO, CGF, CGS,CHA, DAC, DDR, DFT, EAB, EEI, EPI, ERI, ESV, GIS, GOO, HEU, IFY, IHW,IRN, ITE, ITW, JBW, JOZ, JSN, KFI, LEV, LTA, LTJ, MER, MON, MTF, MVY,NSI, OWE, PAU, PHI, RHO, RTE, RTH, RWR, SAS, SAT, SAV, SBN, SFW, SIV,TSC, UEI, UFI, VNI, WEI, WEN, YUG, and ZON, or the like are preferred.In particular, AEI, AFX, CHA, DDR, HEU, LEV, LTA, RHO are preferred dueto ease of zeolite crystallization.

When the framework that forms the pores of the zeolite is configured asa ring of less than or equal to an n-membered ring of oxygen, thearithmetic average of the short diameter and the long diameter of thepores of oxygen n-membered rings is taken as the average pore diameterof the zeolite. An oxygen n-membered ring is simply referred to as ann-membered ring in which the number of oxygen atoms that configure theframework that forms the pore is taken to be a number n, in which atleast one of a Si atom, Al atom and P atom is included, and which is amoiety that forms a ring structure in which the respective oxygen atomsare bound with a Si atom, an Al atom, a P atom, or the like. Forexample, when the zeolite has pores formed from an oxygen 8-memberedring, an oxygen 6-membered ring, an oxygen 5-membered ring, and anoxygen 4-membered ring (that is to say, only has pores that are formedby a ring that is less than or equal to an oxygen 8-membered ring), thearithmetic average of the short diameter and the long diameter of thepores of oxygen 8-membered rings is taken to be the average porediameter.

Furthermore, when the zeolite has plural types of oxygen n-membered ringpores having equal values for n, the arithmetic average of the shortdiameter and the long diameter of the all of the oxygen n-membered ringpores is taken to be the average pore diameter of the zeolite. Forexample, when the zeolite only has pores formed from rings that are lessthan or equal to an oxygen 8-membered ring, and when there is pluraltypes of oxygen 8-membered ring pores, the arithmetic average of theshort diameter and the long diameter of the all of the oxygen 8-memberedring pores is taken as the average pore diameter of the zeolite.

The average pore diameter of the zeolite membrane is uniquely defined bythe framework structure. The average pore diameter of respectiveframework structures may be calculated based on the values disclosed inThe International Zeolite Association (IZA) “Database of ZeoliteStructures” [online], [searched Jan. 22, 2015], Internet <URL:http://www.iza-structure.org/databases/>.

When the framework that forms the pores of the zeolite is formed fromrings of less than or equal to an oxygen n-membered ring, the variationcoefficient of the pore diameter of the zeolite membrane is calculatedusing a standard deviation calculated with reference to a population ofthe short diameters and the long diameters in the oxygen n-membered ringpores. When the zeolite has a plurality of oxygen n-membered ringshaving equal values for n, the variation coefficient is calculated usinga standard deviation calculated with reference to the population ofshort diameters and long diameters of all the oxygen n-membered ringpores. For example, when the zeolite has pores formed from an oxygen8-membered ring, an oxygen 6-membered ring, an oxygen 5-membered ring,and an oxygen 4-membered ring (that is to say, only has pores that areformed by rings of less than or equal to an oxygen 8-membered ring), thevariation coefficient is calculated using a standard deviationcalculated with reference to the population of short diameters and longdiameters of all the oxygen 8-membered ring pores.

When the first separation membrane 30 is a silica membrane, the averagepore diameter and the variation coefficient can be adjusted bycontrolling the type of membrane starting material, the hydrolysisconditions for the membrane starting materials, the firing temperature,and the firing time, or the like. The average pore diameter of thesilica membrane may be calculated based on Formula (1) below. In Formula(1), d_(p) denotes the average pore diameter of the silica membrane, fdenotes the normalized Knudsen permeance, d_(k,i) denotes the diameterof the molecule used in Knudsen diffusion testing, and d_(k,He) denotesthe diameter of a helium molecule.f=(1−d _(k,i) /d _(p))³/(1−d _(k,He) /d _(p))³  (1)

The details of the calculation method for the average pore diameter andin relation to Knudsen diffusion testing are disclosed in Hye Ryeon Lee(four others), “Evaluation and fabrication of pore-size-tuned silicamembranes with tetraethoxydimethyl disiloxane for gas separation”, AlChEJournal, Volume 57. Issue 10, 2755-2765, October 2011.

The variation coefficient of the silica membrane can be calculated withreference to the pore diameter distribution that is measured using anano-perm porometer.

When the first separation membrane 30 is a carbon membrane, the averagepore diameter and the variation coefficient can be adjusted bycontrolling the type of membrane starting material, the firingtemperature, the firing time, and the firing atmosphere, or the like.The average pore diameter of the carbon membrane may be calculated basedon Formula (1) above. The variation coefficient of the carbon membranecan be calculated with reference to a pore diameter distribution that ismeasured using a nano-perm porometer.

The second separation membrane 40 is formed on the first separationmembrane 30. In the present embodiment, a mixed gas that contains atleast methane and nitrogen is brought into contact with the surface ofthe second separation membrane 40. The second separation membrane 40 maybe configured by use of an inorganic material, an organic material, ametal material, a metallo-organic framework (MOF), or a compositematerial of these materials. In consideration of thermal resistantproperties and organic solvent resistant properties, an inorganicmembrane such as a zeolite membrane, silica membrane, and carbonmembrane or the like is suitable as the second separation membrane 40.It is noted that a silica membrane includes an organic silica membranein which an organic functional group is bound to silica.

The second separation membrane 40 is a membrane having a differentmatrix from the first separation membrane 30. That is to say, the firstseparation membrane 30 and the second separation membrane 40 differ inrelation to at least one feature such as the constituent material, theframework structure, the presence/absence of an addition or the additionamount of the metal cation or metal complex as described below, theaverage pore diameter, the variation coefficient of the pore diameter,the permeation coefficient, the separation coefficient, or the like. Inthe present embodiment, the matrix of the second separation membrane 40differs from the first separation membrane 30 in relation to the featurethat the addition amount of the metal cation or metal complex is greaterthan the first separation membrane 30.

The second separation membrane 40 includes addition of at least one of ametal complex that tends to adsorb nitrogen in comparison to methane(referred to below as “nitrogen adsorbing metal complex”) and a metalcation that tends to adsorb nitrogen in comparison to methane (referredto below as “nitrogen adsorbing metal cation”). In this manner, thesecond separation membrane 40 selectively adsorbs nitrogen in a mixedgas that contains at least methane and nitrogen. The second separationmembrane 40 may be termed “a nitrogen selective adsorbing layer”. Thenitrogen adsorbing metal cation may be used at least one elementselected from Sr, Mg. Li, Ba, Ca, Cu, and Fe. The nitrogen adsorbingmetal complex may be used a complex that includes at least one elementselected from Ti, Fe, Ru, Mo, Co and Sm. The addition amount(concentration) and type of the nitrogen adsorbing metal complex or thenitrogen adsorbing metal cation in the second separation membrane 40 canbe measured by EDX (Energy dispersive X-ray spectrometry). Althoughthere is no particular limitation in relation to the total concentrationof the nitrogen adsorbing metal complex or the nitrogen adsorbing metalcation, it may be configured as 0.01 to 60%. In consideration of theadsorbent properties of nitrogen, a value of greater than or equal to0.03% is preferred, and in light of inhibiting pore blockage as a resultof an excessive amount of the nitrogen adsorbing metal complex or thenitrogen adsorbing metal cation, a value of less than or equal to 50% ismore preferred.

In this context, the term in the present embodiment of “tends to adsorbnitrogen when compared to methane” denotes a configuration in whichimmediately after exposure to a mixed gas of 1:1 nitrogen and methane,the adsorption amount of nitrogen is greater than the adsorption amountof methane, that is to say, a configuration in which the adsorptionratio of nitrogen is larger. An adsorption ratio can be obtained bymeasuring the adsorption amount of nitrogen and methane using a powderof the materials that configure the second separation membrane 40.Although there is no particular limitation in relation to the method ofmeasuring the adsorption ratio, for example, a mixed gas containing 1:1nitrogen and methane may be supplied at 10 ml/min to 10 g of a powder ofthe materials that configure the second separation membrane 40, and themolar ratio of nitrogen and methane that is adsorbed by the powder in aninitial period (for example, about 10 minutes) may be measured underpredetermined conditions (room temperature, 0.1 MPa).

The average pore diameter of the second separation membrane 40 ispreferably greater than or equal to the average pore diameter of thefirst separation membrane 30, and more preferably greater than theaverage pore diameter of the first separation membrane 30. The averagepore diameter of the second separation membrane 40 is preferably greaterthan the molecular diameter of methane. In this manner, it is possiblefor the methane that infiltrates the second separation membrane 40 tosmoothly discharge to the outside.

Although there is no particular limitation in relation to the variationcoefficient of the pore diameter of the second separation membrane 40,it may be greater than the variation coefficient of the pore diameter ofthe first separation membrane 30.

When the second separation membrane 40 is a zeolite membrane, there isno particular limitation in relation to the framework structure. Thevariation coefficient of the pore diameter and the average pore diameterof the zeolite membrane are determined by the framework structure of thezeolite.

When the second separation membrane 40 is a carbon membrane or a silicamembrane, the variation coefficient and average pore diameter can beadjusted by controlling the firing (thermal processing) conditions, orthe like. The average pore diameter of the carbon membrane or a silicamembrane can be calculated based on Formula (1) above.

Method of Manufacturing Separation Membrane Structure

A method of manufacturing the separation membrane structure 10 will bedescribed.

(1) Formation of Porous Support 20

Firstly, starting materials for the substrate 21 are molded into adesired shape by use of extrusion molding, a press molding method, aslip cast method, or the like to thereby form a green body for thesubstrate 21. Next, the green body for the substrate 21 is fired (forexample, 900 degrees C. to 1450 degrees C.) to thereby form thesubstrate 21.

Then, an intermediate layer slurry is prepared by use of a ceramicstarting material having a desired particle diameter, and theintermediate layer slurry is formed as a membrane on a surface of thesubstrate 21 to thereby form a green body for the intermediate layer 22.Next, the green body for the intermediate layer 22 is fired (forexample, at 900 degrees C. to 1450 degrees C.) and to thereby form theintermediate layer 22.

Then, a surface layer slurry is prepared by use of a ceramic startingmaterial having a desired particle diameter, and the surface layerslurry is formed as a membrane on a surface of the intermediate layer 22to thereby form a green body for the surface layer 23. Next, the greenbody for the surface layer 23 is fired (for example, at 900 degrees C.to 1450 degrees C.) and to thereby form the surface layer 23.

The porous support 20 is formed in the above manner.

(2) Formation of First Separation Membrane 30

A first separation membrane 30 is formed on a surface of the poroussupport 20. The first separation membrane 30 may be formed using a knownand conventional method depending on the type of membrane. Next, therespective methods for forming a zeolite membrane, a silica membrane anda carbon membrane will be described as examples of a method of formingthe first separation membrane 30.

Zeolite Membrane

Firstly, after pre-coating zeolite as a seed crystal on the surface ofthe surface layer 23, the porous support 20 is immersed inside apressure-resistant vessel containing a starting material solution thatincludes a silica source, an alumina source, an organic template, analkali source and water or the like.

Next, the pressure-resistant vessel is placed in a drying oven andsubjected to thermal processing (hydrothermal synthesis) for about 1 to240 hours at 100 to 200 degrees C. to thereby form a zeolite membrane.Next, the porous support 20 formed the zeolite membrane is washed anddried at 80 to 100 degrees C.

Then, in a configuration in which an organic template is included in thestarting material solution, the porous support 20 is placed in anelectric furnace, and heated in an atmosphere of air at 400 to 800degrees C. for 1 to 200 hours to thereby combust and remove the organictemplate.

The average pore diameter and variation coefficient of a zeolitemembrane formed in the above manner is uniquely determined by theframework structure of the zeolite.

Silica Membrane

Firstly, an alkoxysilane such as tetraethoxysilane or the like, anorganic alkoxysilane such as methyltrimethoxysilane or the like, or anorganic hydroxysilane such as a carboxyethyl silane triol sodium salt orthe like is subjected to hydrolysis or condensation in the presence of acatalyst such as hydrochloric acid, nitric acid or the like to therebyform a sol solution, and is diluted with ethanol or water to therebyprepare a precursor solution (silica sol solution).

Then, after the precursor solution is brought into contact with thesurface of the surface layer 23, the surface layer 23 is heated to 400to 700 degrees C. at a rate of 100 degrees C./hr and maintained for onehour. Then the temperature is allowed to fall at a rate of 100 degreesC./hr. A silica membrane is formed by 3 to 5 repetitions of the abovesteps.

The average pore diameter and variation coefficient of a silica membraneformed in the above manner can be adjusted by controlling the hydrolysisconditions, the firing temperature, the firing time, or the like.

Carbon Membrane

Firstly, a thermo-curing resin such as an epoxy resin, polyimide resin,or the like, a thermoplastic resin such as polyethylene or the like, acellulose resin, or precursor materials for these materials is dissolvedin water or an organic solvent such as methanol, acetone,tetrahydrofuran, NMP, toluene, or the like to thereby prepare aprecursor solution.

Then, after the precursor solution is brought into contact with thesurface of the surface layer 23, thermal processing (for example, 500 to1000 degrees C.) is performed depending on the type of resin containedin the precursor solution to thereby form a carbon membrane.

The average pore diameter and variation coefficient of a carbon membraneformed in the above manner can be adjusted by controlling the type ofresin, the thermal processing temperature, the thermal processing time,the thermal processing atmosphere, or the like.

(3) Formation of Second Separation Membrane 40

A second separation membrane 40 is formed on a surface of the firstseparation membrane 30. The second separation membrane 40 may be formedusing a known and conventional method depending on the type of membrane.Therefore the second separation membrane 40 is formed depending on themethod of formation used in relation to the first separation membrane30. Next, the method of forming a zeolite membrane, a silica membraneand a carbon membrane will be described in sequence.

Zeolite Membrane

Firstly, after pre-coating zeolite as a seed crystal on the surface ofthe first separation membrane 30, the porous support 20 is immersedinside a pressure-resistant vessel containing a starting materialsolution that adds at least one of a nitrogen adsorbing metal cation ora nitrogen adsorbing metal complex to a silica source, an aluminasource, an organic template, an alkali source and water. At that time,the addition amount of the nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex may be adjusted to thereby enablecontrol of the nitrogen adsorbing properties of the second separationmembrane 40. Coating with a seed crystal may be omitted.

Next, the pressure-resistant vessel is placed in a drying oven andsubjected to thermal processing (hydrothermal synthesis) for about 1 to240 hours at 100 to 200 degrees C. to thereby form a zeolite membrane.Next, the porous support 20 formed the zeolite membrane is washed anddried at 80 to 100 degrees C.

Then, when an organic template is included in the starting materialsolution, the porous support 20 is placed in an electric furnace, andheated in an atmosphere of air at 400 to 800 degrees C. for 1 to 200hours to thereby combust and remove the organic template.

The nitrogen adsorbing metal cation or the nitrogen adsorbing metalcomplex may be introduced into the zeolite membrane after membraneformation by use of a method such as ion exchange or immersion, or thelike rather than by addition in advance into the starting materialsolution. Furthermore, the nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex may be introduced into the zeolitemembrane after membrane formation by use of a method such as ionexchange, immersion, or the like in combination with addition in advanceinto the starting material solution. In this context, it is possible tocontrol the nitrogen adsorbing properties of the second separationmembrane 40 by adjusting the introduction amount of the nitrogenadsorbing metal cation or the nitrogen adsorbing metal complex.

The average pore diameter of a zeolite membrane formed in the abovemanner is uniquely determined by the framework structure of the zeolite.

Silica Membrane

Firstly, an alkoxysilane such as tetraethoxysilane or the like, anorganic alkoxysilane such as methyltrimethoxysilane or the like, or anorganic hydroxysilane such as a carboxyethyl silane triol sodium salt orthe like is subjected to hydrolysis or condensation in the presence ofnitric acid to thereby form a sol solution, and is diluted with ethanolor water that includes addition of at least one of a nitrogen adsorbingmetal cation or a nitrogen adsorbing metal complex to thereby prepare aprecursor solution (silica sol solution). In this context, it ispossible to control the nitrogen adsorbing properties of the secondseparation membrane 40 by adjusting the addition amount of the nitrogenadsorbing metal cation.

Then, after the precursor solution is brought into contact with thesurface of the first separation membrane 30, the surface layer 23 isheated to 400 to 700 degrees C. at a rate of 100 degrees C./hr andmaintained for one hour. Then the temperature is allowed to fall at arate of 100 degrees C./hr. A silica membrane is formed by 3 to 5repetitions of the above steps.

The nitrogen adsorbing metal cation or the nitrogen adsorbing metalcomplex may be introduced into the silica membrane after membraneformation by use of a method such as ion exchange or immersion, or thelike rather than by addition in advance into the precursor solution.Furthermore, the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex may be introduced into the silica membrane aftermembrane formation by use of a method such as ion exchange, immersion,or the like in combination with addition in advance into the precursorsolution. In this context, it is possible to control the nitrogenadsorbing properties of the second separation membrane 40 by adjustingthe introduction amount of the nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex.

The average pore diameter and variation coefficient of a silica membraneformed in the above manner can be adjusted by controlling the hydrolysisconditions, the firing temperature, the firing time, or the like.

Carbon Membrane

Firstly, a thermo-curing resin such as an epoxy resin, polyimide resin,or the like, a thermoplastic resin such as polyethylene or the like, acellulose resin, or precursor materials for these materials is dissolvedin water or an organic solvent such as methanol, acetone,tetrahydrofuran, NMP, toluene, or the like that includes addition of atleast one of a nitrogen adsorbing metal cation or a nitrogen adsorbingmetal complex to thereby prepare a precursor solution. In this context,it is possible to control the nitrogen adsorbing properties of thesecond separation membrane 40 by adjusting the addition amount of thenitrogen adsorbing metal cation.

Then, after the precursor solution is brought into contact with thesurface of the first separation membrane 30, thermal processing (forexample, 500 to 1000 degrees C.) is performed depending on the type ofresin contained in the precursor solution to thereby form a carbonmembrane.

The nitrogen adsorbing metal cation or the nitrogen adsorbing metalcomplex may be introduced into the carbon membrane after membraneformation by use of a method such as ion exchange or immersion, or thelike rather than by addition in advance into the precursor solution.Furthermore, the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex may be introduced into the carbon membrane aftermembrane formation by use of a method such as ion exchange, immersion,or the like in combination with addition in advance into the precursorsolution. In this context, it is possible to control the nitrogenadsorbing properties of the second separation membrane 40 by adjustingthe introduction amount of the nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex.

The average pore diameter and variation coefficient of a carbon membraneformed in the above manner can be adjusted by controlling the type ofresin, the thermal processing temperature, the thermal processing time,the thermal processing atmosphere, or the like.

Operation and Effect

The separation membrane structure 10 according to the above embodimentincludes the porous support 20, a first separation membrane 30 that isformed on the porous support 20, and a second separation membrane 40that is formed on the first separation membrane 30. The first separationmembrane 30 has an average pore diameter of greater than or equal to0.32 nm and less than or equal to 0.44 nm. The second separationmembrane 40 includes addition of at least one of a metal cation or metalcomplex that exhibits a tendency to adsorb nitrogen in comparison tomethane.

Therefore, it is possible to increase the nitrogen partial pressure inthe second separation membrane 40 by an adsorption effect in the secondseparation membrane 40 and to enable selective permeation of nitrogen bya molecular sieve effect in the first separation membrane 30. As aresult, it is possible to efficiently separate methane and nitrogen byenabling both satisfactory separation properties and permeationproperties of nitrogen.

Other Embodiments

Although an embodiment of the present invention has been described, thepresent invention is not limited to the above embodiment, and variousmodifications are possible within a scope that does not depart from thespirit of the invention.

For example, although the porous support 20 includes the substrate 21,the intermediate layer 22 and the surface layer 23, one or both of theintermediate layer 22 and the surface layer 23 may be omitted.

Furthermore, although the separation membrane structure 10 includes thefirst separation membrane 30 and the second separation membrane 40stacked onto the porous support 20, a functional layer or protectivelayer may be further provided in a stacked configuration onto the secondseparation membrane 40. This type of functional layer or protectivelayer may be an inorganic layer such a zeolite layer, carbon layer,silica layer, or the like, or may be an organic layer such as apolyimide layer, silicone layer or the like.

Furthermore, although the nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex is not actively added to the firstseparation membrane 30, the first separation membrane 30 may include atleast one of the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex. The nitrogen adsorbing metal cation or thenitrogen adsorbing metal complex may be present in a greater quantity ina region of the first separation membrane 30 near to the secondseparation membrane 40. The term “region of the first separationmembrane 30 near to the second separation membrane 40” is a regionwithin a 50% of the thickness of the first separation membrane 30 fromthe interface with the second separation membrane 40. The method bywhich the nitrogen adsorbing metal cation or the nitrogen adsorbingmetal complex is included in the first separation membrane 30 includes amethod in which the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex is added in advance to a starting materialsolution for the first separation membrane 30, or a method in whichthere is introduction into the first separation membrane 30 at the sametime as the nitrogen adsorbing metal cation or the nitrogen adsorbingmetal complex is introduced into the second separation membrane 40 byion exchange, immersion or the like.

Nitrogen can be smoothly drawn from the second separation membrane 40into the first separation membrane 30 as a result of inclusion of thenitrogen adsorbing metal cation or the nitrogen adsorbing metal complexin the first separation membrane 30, and therefore it is possible tofurther enhance the separation properties and permeation properties ofnitrogen.

The concentration of the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex in the first separation membrane 30 ispreferably lower than that of the second separation membrane 40. Theconcentration of the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex in the first separation membrane 30 is morepreferably less than or equal to 70% of the concentration in the secondseparation membrane 40, and still more preferably less than or equal to50%. When the nitrogen adsorbing metal cation or the nitrogen adsorbingmetal complex is present in a larger quantity in a region of the firstseparation membrane 30 that is near to the second separation membrane40, the concentration in a region of the first separation membrane 30that is near to the second separation membrane 40 is preferably lowerthan the concentration in the overall second separation membrane 40.

In this manner, nitrogen can be smoothly transferred from the firstseparation membrane 30 into the porous support 20 by configuring a lowconcentration of the nitrogen adsorbing metal cation or the nitrogenadsorbing metal complex in the first separation membrane 30.

EXAMPLES

The examples of the present invention will be described below. However,the present invention is not thereby limited to the following examples.

Preparation of Sample No. 1

A separation membrane structure according to Sample No. 1 is prepared inthe following manner.

Firstly, a tube-shaped porous alumina substrate having a diameter of 10mm and a length of 30 mm is prepared. The pore diameter that opens ontoan outer surface of the porous alumina substrate is 0.1 microns.

Next, a high silica DDR-type zeolite membrane having an Si/Al atomicratio of greater than or equal to 200 is formed as a first separationmembrane on an outer surface of a porous alumina substrate. Morespecifically, firstly, a DDR-type zeolite seed crystal (Si/Al atomicratio ≥200) is diluted with ethanol, and the seeding slurry solutionprepared to have a concentration of 0.1 mass % is caused to flow intothe cell of the porous alumina substrate, and the inner portion of thecell is air-dried under predetermined conditions (room temperature, airvelocity 5 m/s, 10 min). Next, after placing 7.35 g of ethylene diamine(manufactured by Wako Pure Chemical Industries, Ltd.) in a fluororesinwide-mouthed container, 1.16 g of 1-adamantane amine (manufactured byAldrich) is added and dissolved so that there is no residual precipitateof 1-adamantane amine. Then, 98.0 g of 30 wt % silica sol (trade name:Snowtex S, manufactured by Nissan Chemical Industries, Ltd.) and 116.5 gof distilled water are added to a separate container, stirred gently,and then added to the wide-mouth container and mixed by strong shakingto thereby prepare a sol for membrane formation. The porous aluminasubstrate with the DDR-type zeolite seed crystals attached is placed ina fluororesin inner cylinder (internal volume 300 ml) of a stainlesssteel pressure vessel, the membrane formation sol added and thermalprocessing is performed (hydrothermal synthesis: 130 degrees C., 10hours) to thereby form a high silica DDR-type zeolite membrane. Then,the porous alumina substrate is washed and dried for greater than orequal to 12 hours at 80 degrees C. Next, the porous alumina substrate isheated to 450 degrees C. in an electric furnace and retained for 50hours to thereby combust and remove the 1-adamantane amine from the highsilica DDR-type zeolite membrane. The average pore diameter of the highsilica DDR-type zeolite membrane is 0.40 nm and the variationcoefficient is 0.14.

Next, a low silica DDR-type zeolite membrane having an Si/Al atomicratio of 40 is formed as a second separation membrane on the surface ofthe first separation membrane. More specifically, firstly, after placing152.4 g of distilled water in a fluororesin wide-mouthed container, 1.32g of 1-adamantane amine (manufactured by Aldrich), 0.35 g sodiumhydroxide (manufactured by Sigma-Aldrich), 52.6 g of 30 wt % silica sol(Trade name Snowtex S, Nissan chemical Industries, Ltd.) and 0.36 g ofsodium aluminate acid (manufactured by Wako Pure Chemical Industries,Ltd.) are added and stirred to thereby prepare a membrane formation sol.Next, after the porous alumina substrate formed the high silica DDR-typezeolite membrane is placed in a fluororesin inner cylinder (internalvolume 300 ml) of a stainless steel pressure vessel, the formulatedstarting material solution is added and thermal processing is performed(hydrothermal synthesis: 160 degrees C., 10 hours) to thereby form a lowsilica DDR-type zeolite membrane. Then, the porous alumina substrate iswashed and dried for greater than or equal to 12 hours at 80 degrees C.Next, the porous alumina substrate is heated to 450 degrees C. in anelectric furnace and retained for 50 hours to thereby combust and removethe 1-adamantane amine from the low silica DDR-type zeolite membrane.The average pore diameter of the low silica DDR-type zeolite membrane is0.40 nm.

Then, Li introduced as a metal cation into the second separationmembrane by adding lithium chloride (manufactured by Kanto Kagaku) towater to achieve 0.1 mol/L and maintaining the formulated Li ionexchange solution in contact with the second separation membrane for 24hours. Thereafter the second separation membrane is rinsed with waterand dried (70 degrees C., 12 hours).

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 2

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, a low silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a second separation membrane on a surface of thefirst separation membrane.

Next strontium nitrate (manufactured by Wako Pure Chemical Industries,Ltd.) is added to water to achieve 0.1 mol/L and the formulated Sr ionexchange solution is maintained in contact with the second separationmembrane for 24 hours to thereby introduce Sr as a metal cation into thesecond separation membrane. Thereafter the second separation membrane isrinsed with water and dried (70 degrees C., 12 hours).

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 3

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, a low silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a second separation membrane on a surface of thefirst separation membrane.

Next barium chloride dihydrate (manufactured by Wako Pure ChemicalIndustries, Ltd.) is added to water to achieve 0.1 mol/L and theformulated Ba ion exchange solution is maintained in contact with thesecond separation membrane for 24 hours to thereby introduce Ba as ametal cation into the second separation membrane. Thereafter the secondseparation membrane is rinsed with water and dried (70 degrees C., 12hours).

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 4

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, a low silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a second separation membrane on a surface of thefirst separation membrane.

Next copper chloride (manufactured by Wako Pure Chemical Industries,Ltd.) is added to water to achieve 0.1 mol/L and the formulated Cu ionexchange solution is maintained in contact with the second separationmembrane for 24 hours to thereby introduce Cu as a metal cation into thesecond separation membrane. Thereafter the second separation membrane isrinsed with water and dried (70 degrees C., 12 hours), and heating isperformed in a vacuum to reduce to the Cu to a monovalent configuration.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 5

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, an organic silica membrane with large pore diameter is formed as asecond separation membrane on a surface of the first separationmembrane. More specifically, 24.0 g of a 25% aqueous solution of acarboxyethyl silane triol sodium salt, 73.0 g of distilled water, and3.0 g of 60% nitric acid are added and stirred using a magnetic stirrer(60 degrees C., 6 hours) to thereby form a coating solution. Aftercoating and drying the coating solution on the surface of the firstseparation membrane, firing is performed in an air for two hours at 300degrees C. Separately, after coating and drying the coating solution onthe porous alumina substrate, the pore diameter of a sample in whichfiring is performed in an air for two hours at 300 degrees C. isdetermined to be 0.45 nm. Therefore, the pore diameter of the secondseparation membrane is determined to be 0.45 nm.

Next Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 6

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as a second separation membrane on asurface of the first separation membrane.

Next Sr is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 2.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 7

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as a second separation membrane on asurface of the first separation membrane.

Next an Fe complex solution that dissolves [1,2-bis (diphenylphosphino)ethane] iron dichloride as an Fe complex in tetrahydrofuran (THF) ismaintained in contact with the second separation membrane for 24 hoursto thereby introduce an Fe complex into the second separation membrane.Thereafter the second separation membrane is rinsed with water and dried(70 degrees C., 12 hours).

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 8

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as a second separation membrane on asurface of the first separation membrane.

Next a Mn complex solution that dissolves (cyclopentadienyl) manganesetricarbonyl as an Mn complex in benzene is maintained in contact withthe second separation membrane for 24 hours to thereby introduce a Mncomplex into the second separation membrane. Thereafter the secondseparation membrane is rinsed with water and dried (70 degrees C., 12hours), and ultraviolet irradiation is performed to form a dicarbonylconfiguration of the Mn complex.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 9

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica DDR-type zeolite membrane that is the same as SampleNo. 1 is formed as a first separation membrane on an outer surface ofthe porous alumina substrate.

Next, an organic silica membrane with small pore diameter is formed as asecond separation membrane on a surface of the first separationmembrane. More specifically, 24.0 g of a 25% aqueous solution ofcarboxyethyl silane triol sodium salt, 73.0 g of distilled water, and3.0 g of 60% nitric acid are added and stirred using a magnetic stirrer(60 degrees C., 6 hours) to thereby form a coating solution. Aftercoating and drying the coating solution on the surface of the firstseparation membrane, firing is performed in an air for two hours at 150degrees C. Separately, after coating and drying the coating solution onthe porous alumina substrate, the pore diameter of a sample in whichfiring is performed in an air for two hours at 150 degrees C. isdetermined to be 0.30 nm. Therefore, the pore diameter of the secondseparation membrane is determined to be 0.30 nm.

Next Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 10

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica CHA-type zeolite membrane having an Si/Al atomicratio of 17 is formed making reference to Japanese Patent ApplicationLaid-Open No. 2013-126649 as a first separation membrane on an outersurface of the porous alumina substrate. The average pore diameter ofthe high silica CHA-type zeolite membrane is 0.38 nm and the variationcoefficient is 0.00.

Then, a low silica CHA-type zeolite membrane having an Si/Al atomicratio of 5 is formed making reference to Japanese Patent ApplicationLaid-Open No. 2013-126649 as a second separation membrane on a surfaceof the first separation membrane. The average pore diameter of the lowsilica CHA-type zeolite membrane is 0.38 nm.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 11

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, an AFX-type zeolite membrane is formed as a first separationmembrane on an outer surface of the porous alumina substrate. Morespecifically, making reference to Chemistry of Materials, 8(10),2409-2411 (1996), an AFX-type zeolite powder is synthesized and coatedonto an outer surface of the porous alumina substrate. Then, the porousalumina substrate is immersed in a synthesis sol that is the same as theconfiguration used in relation to the synthesis of the zeolite powder tothereby perform membrane formation of an AFX-type zeolite membrane byhydrothermal synthesis. The average pore diameter of the AFX-typezeolite membrane is 0.35 nm and the variation coefficient is 0.04.

Then, an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as the second separation membrane on thesurface of the first separation membrane.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 12

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, an HEU-type zeolite membrane is formed as a first separationmembrane on an outer surface of the porous alumina substrate. Morespecifically, making reference to Japanese Patent Application Laid-OpenNo. 2000-237584, an HEU-type zeolite powder is synthesized and coatedonto an outer surface of the porous alumina substrate. Then, the porousalumina substrate is immersed in a synthesis sol that is the same as theconfiguration used in relation to the synthesis of the zeolite powder tothereby perform membrane formation of an HEU-type zeolite membrane byhydrothermal synthesis. The average pore diameter of the HEU-typezeolite membrane is 0.43 nm and the variation coefficient is 0.39.

Then an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as the second separation membrane on thesurface of the first separation membrane.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 13

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, an SAT-type zeolite membrane is formed as a first separationmembrane on an outer surface of the porous alumina substrate. Morespecifically, making reference to Journal of the Chemical Society,Dalton Transactions, 1997, 4485-4490, an SAT-type zeolite powder issynthesized and coated onto an outer surface of the porous aluminasubstrate. Then, the porous alumina substrate is immersed in a synthesissol that is the same as the configuration used in relation to thesynthesis of the zeolite powder to thereby perform membrane formation ofan SAT-type zeolite membrane by hydrothermal synthesis. The average porediameter of the SAT-type zeolite membrane is 0.43 nm and the variationcoefficient is 0.42.

Then an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as the second separation membrane on thesurface of the first separation membrane.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 14

Firstly, a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, an ANA-type zeolite membrane is formed as a first separationmembrane on an outer surface of the porous alumina substrate. Morespecifically, making reference to Japanese Patent Application Laid-OpenNo. 54-146300, an ANA-type zeolite powder is synthesized and coated ontoan outer surface of the porous alumina substrate. Then, the porousalumina substrate is immersed in a synthesis sol that is the same as theconfiguration used in relation to the synthesis of the zeolite powder tothereby perform membrane formation of an ANA-type zeolite membrane byhydrothermal synthesis. The average pore diameter of the ANA-typezeolite membrane is 0.29 nm and the variation coefficient is 0.63.

Then an organic silica membrane with large pore diameter that is thesame as Sample No. 5 is formed as the second separation membrane on thesurface of the first separation membrane.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Preparation of Sample No. 15

Firstly a porous alumina substrate that is the same as Sample No. 1 isprepared.

Next, a high silica MFI-type zeolite membrane having an Si/Al atomicratio of greater than or equal to 200 is formed as a first separationmembrane on an outer surface of the porous alumina substrate. Morespecifically, firstly, an MFI-type zeolite seed crystal (Si/Al atomicratio ≥200) is diluted with ethanol, the seeding slurry solutionprepared to have a concentration of 0.1 mass % is caused to flow intothe cell of the porous alumina substrate, and the inner portion of thecell is air-dried under predetermined conditions (room temperature, airvelocity 5 m/s, 10 min). Next, after mixing 0.86 g of 40 mass %tetrapropylammonium hydroxide solution (manufactured by SACHEM, Inc.)and 0.45 g of tetrapropylammonium bromide (manufactured by Wako PureChemical Industries), 192.0 g of distilled water and 6.75 g of about 30mass % silica sol (trade name: Snowtex S, manufactured by Nissanchemical Industries, Ltd.) are added and stirred with a magnetic stirrer(room temperature, 30 minutes) to thereby prepare a sol for membraneformation. The resulting membrane formation sol is placed in afluororesin inner cylinder (internal volume 300 ml) of a stainless steelpressure vessel and then the porous alumina substrate with MFI-typezeolite seed crystals attached is immersed and reacted for 20 hours in ahot air drying oven at a temperature of 160 degrees C. to thereby form ahigh silica MFI-type zeolite membrane. Then, the porous aluminasubstrate is washed and dried for greater than or equal to 12 hours at80 degrees C. Next, the porous alumina substrate is heated to 500degrees C. in an electric furnace and retained for 4 hours to remove thetetrapropylammonium from the high silica MFI-type zeolite membrane. Theaverage pore diameter of the high silica MFI-type zeolite membrane is0.54 nm and the variation coefficient is 0.04.

Next, a low silica MFI-type zeolite membrane having an Si/Al atomicratio of 20 is formed as a second separation membrane on the surface ofthe first separation membrane. More specifically, firstly, afterdiluting a low silica MFI-type zeolite seed crystal (Si/Al atomicratio=20) with ethanol, the seeding slurry solution prepared to have aconcentration of 0.1 mass % is caused to flow into the cell of theporous alumina substrate, and the inner portion of the cell is air-driedunder predetermined conditions (room temperature, air velocity 5 m/s, 10min). Next, after mixing 6.28 g of 40 mass % tetrapropylammoniumhydroxide solution (manufactured by SACHEM, Inc.), 4.97 g oftetrapropylammonium bromide (manufactured by Wako Pure ChemicalIndustries), 26.3 g of sodium hydroxide (manufactured by Sigma-Aldrich),and 0.54 g of aluminum sulfate (manufactured by Wako Pure ChemicalIndustries), 147.1 g of distilled water and 14.8 g of about 30 mass %silica sol (trade name: Snowtex S, manufactured by Nissan chemicalIndustries, Ltd.) are added and stirred with a magnetic stirrer (roomtemperature, 30 minutes) to thereby prepare a sol for membraneformation. After the resulting membrane formation sol is placed in afluororesin inner cylinder (internal volume 300 ml) of a stainless steelpressure vessel, and the porous alumina substrate with the zeolite seedcrystals attached is immersed and reacted for 32 hours in a hot airdrying oven at a temperature of 160 degrees C. to thereby form a lowsilica MFI-type zeolite membrane. Then, the porous alumina substrate iswashed and dried for greater than or equal to 12 hours at 80 degrees C.Next, the porous alumina substrate is heated to 500 degrees C. in anelectric furnace and retained for 4 hours to remove thetetrapropylammonium from the low silica MFI-type zeolite membrane. Theaverage pore diameter of the low silica MFI-type zeolite membrane is0.54 nm.

Next, Li is introduced as a metal cation into the second separationmembrane in the same manner as Sample No. 1.

Next, one end of the porous alumina substrate is sealed by adhering aglass plate with an epoxy resin to one end of the porous aluminasubstrate. Then, a glass tube is connected with an epoxy resin to theother end of the porous alumina substrate.

Gas Separation Testing

Gas separation testing is performed by use of the separation membranestructure in Sample Nos. 1 to 15.

Firstly after sufficiently drying the separation membrane structure, amixed gas of nitrogen and methane (molar ratio 1:1) is supplied to anouter side of the separation membrane structure at a temperature of 23degrees C. and a pressure of 0.3 MPa.

Next the composition and flow amount of a permeating gas that permeatesthe first separation membrane and the second separation membrane andflows out of the glass tube is analyzed. The flow amount of thepermeating gas is measured using a mass flow meter. The composition ofthe permeating gas is measured using gas chromatography. The compositionand flow amount of the permeating gas are used to calculate a permeationrate of methane and nitrogen per unit membrane surface area

unit pressure difference

unit membrane thickness, and (nitrogen permeation rate)/(methanepermeation rate) is taken to be the nitrogen separation performance.Table 1 shows an evaluation of the nitrogen separation performance intoA, B, C from highest to lowest, and evaluates the high nitrogenpermeation rate into A, B, C from highest to lowest.

TABLE 1 First Separation Membrane Second Separation Membrane MetalCation Nitrogen Nitrogen Average Pore Variation Average Pore ofSeparation Permeation Sample No. Matarial Diameter (nm) CoefficientMatarial Diameter (nm) Metal Complex Performance Rate 1 High Silica DDR0.40 0.14 Low Silica DDR 0.40 Li A B 2 High Silica DDR 0.40 0.14 LowSilica DDR 0.40 Sr A B 3 High Silica DDR 0.40 0.14 Low Silica DDR 0.40Ba A B 4 High Silica DDR 0.40 0.14 Low Silica DDR 0.40 Cu A B 5 HighSilica DDR 0.40 0.14 Organic Silica with 0.45 Li A A Large Pore Diameter6 High Silica DDR 0.40 0.14 Organic Silica with 0.45 Sr A A Large PoreDiameter 7 High Silica DDR 0.40 0.14 Organic Silica with 0.45 Fe ComplexA A Large Pore Diameter 8 High Silica DDR 0.40 0.14 Organic Silica with0.45 Mn Complex A A Large Pore Diameter 9 High Silica DDR 0.40 0.14Organic Silica with 0.30 Li B B Small Pore Diameter 10 High Silica CHA0.38 0.00 Low Silica CHA 0.33 Li A B 11 AFX 0.35 0.04 Organic Silicawith 0.45 Li A A Large Pore Diameter 12 HEU 0.43 0.39 Organic Silicawith 0.45 Li A B Large Pore Diameter 13 SAT 0.43 0.42 Organic Silicawith 0.45 Li B B Large Pore Diameter 14 ANA 0.29 0.63 Organic Silicawith 0.45 Li C C Large Pore Diameter 15 High Silica MFI 0.54 0.04 LowSilica MFI 0.54 Li C A

As shown in Table 1, the nitrogen separation performance is enhanced inSample Nos. 1 to 13 in which the average pore diameter of the firstseparation membrane is greater than or equal to 0.32 nm and less than orequal to 0.44 nm, and which are provided with a second separationmembrane that includes addition of at least one of a metal cation andmetal complex that exhibit a tendency to adsorb nitrogen in comparisonto methane.

Furthermore, with reference to Samples Nos. 1 to 9 that are providedwith a high silica DDR-type zeolite membrane as the first separationmembrane, nitrogen separation performance exhibits further enhancementin Sample Nos. 1 to 8 in which the average pore diameter of the secondseparation membrane is greater than or equal to the pore diameter of thefirst separation membrane. In addition, with reference to Sample Nos. 1to 8, a further enhancement to the nitrogen permeation rate is exhibitedby Sample Nos. 5 to 8 in which the average pore diameter of the secondseparation membrane is greater than the pore diameter of the firstseparation membrane.

Furthermore, with reference to Sample Nos. 5 to 8, and 11 to 13 thathave the same second separation membrane, a further enhancement to thenitrogen permeation rate is exhibited by Sample Nos. 5 to 8, 11 and 12in which the variation coefficient is less than or equal to 0.4.

The invention claimed is:
 1. A separation membrane structure configuredfor selective permeation of nitrogen from a mixed gas that contains atleast methane and nitrogen, the separation membrane structurecomprising: a porous support, a first separation membrane formed on theporous support, and having an average pore diameter of greater than orequal to 0.32 nm and less than or equal to 0.44 nm, a second separationmembrane formed on the first separation membrane, and including additionof at least one of a metal cation or a metal complex that tends toadsorb nitrogen in comparison to methane.
 2. The separation membranestructure according to claim 1, wherein an average pore diameter of thesecond separation membrane is greater than or equal to the average porediameter of the first separation membrane.
 3. The separation membranestructure according to claim 1, wherein an average pore diameter of thesecond separation membrane is greater than the average pore diameter ofthe first separation membrane.
 4. The separation membrane structureaccording to claim 1, wherein the metal cation added to the secondseparation membrane is at least one element selected from Sr, Mg, Li,Ba, Ca, Cu, and Fe.
 5. The separation membrane structure according toclaim 1, wherein the metal complex added to the second separationmembrane is a complex that includes at least one element selected fromTi, Fe, Ru, Mo, Co and Sm.
 6. The separation membrane structureaccording to claim 1, wherein a variation coefficient obtained bydividing a standard deviation of pore diameters of the first separationmembrane by the average pore diameter of the first separation membraneis less than or equal to 0.4.